Fuel tanks containing diesel fuel or other fuel oil are subject to the accumulation of water in the bottom of the tank. Since the water does not mix with the fuel oil it must occasionally be drained from the tank. In some cases to assure that water does not reach the engine or other utilization device a fuel conditioner is employed to separate the water from the fuel in a filter-like canister. In either case, the tank or the canister should be equipped with means for determining when the water level reaches an unacceptable height and for producing a signal indicating that the high water level has been reached and the water should be removed from the container. A desirable method for removing the water is by a siphon tube extending to the bottom of the container which allows the water to be removed by suction.
Known devices for detecting water level in a fuel container include resistive probes and capacitive probes. The resistive probes depend upon insulating qualities of the fuel oil to establish a high resistive impedance between a probe and the walls of the container, and if water is present in sufficient quantities to contact the probe then a low resistance path between the probe and the container will be established. A circuit is provided for detecting the low resistance due to the presence of water and providing a suitable output indication. A problem with the resistive probe is that the fuel oil tends to form a nonconducting layer of oil or a deposit from the oil, and in addition, there is a tendency for a bacterial sludge to grow on the probe surface at the water/oil interface. The insulating coating on the probe thus prevents its proper operation as a resistive probe and its function deteriorates as the coating builds up over a period of usage. A capacitive probe, for example, as disclosed in the U.S. Pat. No. 4,316,174, to Sutton et al has a dielectric coating so that the conductive portion of the probe is one plate of a capacitor and conductive water surrounding a portion of the probe is the other plate. The build up of an additional dielectric coating decreases the capacitance of the probe but it is still operable. However, the capacitive probe has a drawback in that the probe must have a dielectric coating of high integrity. That is, no pin holes or other resistive leaks are allowable so that when the probe is new or has just been cleaned it will maintain its capacitive characteristics. As a practical matter, this limits the probes to very simple geometries which allow a high quality coating to be applied. A tube, for example, is difficult to coat internally. Even in the case of simple geometries the high quality dielectric coating is an expensive aspect of probe fabrication.